You can define mail aliases in a file named `~/.mailrc'.
These are short mnemonic names which stand for mail addresses or groups of
mail addresses. Like many other mail programs, Emacs expands aliases
when they occur in the `To', `From', `CC', `BCC', and
`Reply-to' fields, plus their `Resent-' variants.

To define an alias in `~/.mailrc', write a line in the following
format:

alias shortaddressfulladdresses

Here fulladdresses stands for one or more mail addresses for
shortaddress to expand into. Separate multiple addresses with
spaces; if an address contains a space, quote the whole address with a
pair of double-quotes.

For instance, to make maingnu stand for
gnu@gnu.org plus a local address of your own, put in
this line:

alias maingnu gnu@gnu.org local-gnu

Emacs also recognizes include commands in `.mailrc' files.
They look like this:

source filename

The file `~/.mailrc' is used primarily by other mail-reading
programs; it can contain various other commands. Emacs ignores
everything in it except for alias definitions and include commands.

Another way to define a mail alias, within Emacs alone, is with the
define-mail-alias command. It prompts for the alias and then the
full address. You can use it to define aliases in your `.emacs'
file, like this:

(define-mail-alias "maingnu" "gnu@gnu.org")

define-mail-alias records aliases by adding them to a
variable named mail-aliases. If you are comfortable with
manipulating Lisp lists, you can set mail-aliases directly. The
initial value of mail-aliases is t, which means that
Emacs should read `.mailrc' to get the proper value.

You can specify a different file name to use instead of
`~/.mailrc' by setting the variable
mail-personal-alias-file.

Normally, Emacs expands aliases when you send the message. You do not
need to expand mail aliases before sending the message, but you can
expand them if you want to see where the mail will actually go. To do
this, use the command M-x expand-mail-aliases; it expands all mail
aliases currently present in the mail headers that hold addresses.

If you like, you can have mail aliases expand as abbrevs, as soon as
you type them in (see section Abbrevs). To enable this feature, execute the
following:

(add-hook 'mail-setup-hook 'mail-abbrevs-setup)

This can go in your `.emacs' file. See section Hooks. If you use this
feature, you must use define-mail-abbrev instead of
define-mail-alias; the latter does not work with this package.
Note that the mail abbreviation package uses the variable
mail-abbrevs instead of mail-aliases, and that all alias
names are converted to lower case.

The mail abbreviation package also provides the C-c C-a
(mail-interactive-insert-alias) command, which reads an alias
name (with completion) and inserts its definition at point. This is
useful when editing the message text itself or a header field such as
`Subject' in which Emacs does not normally expand aliases.

Note that abbrevs expand only if you insert a word-separator character
afterward. However, you can rebind C-n and M-> to cause
expansion as well. Here's how to do that: